Fluorination of polyethylene and other polymeric materials to improve their resistance to solvents and to vapor permeation has long been practiced. Early work was reported by Joffre in U.S. Pat. No. 2,811,468 and Dixon et al. in U.S. Pat. No. 3,862,284. The '468 patent discloses a process of fluorinating polyethylene material in a chamber to improve barrier properties thus enhancing the material as a wrapping material for foodstuffs and perishable materials. More specifically, it discloses a fluorination process carried out at room temperature or at a temperature below about 50.degree. C. with 100% fluorine or an inert gas containing 10% fluorine for a period of about 20 to 150 minutes to achieve surface fluorine concentrations of 0.03 to 3.5 percent by weight of fluorine based on the weight of the polyethylene. Dixon et al., in '284 discloses fluorination of a variety of polymeric materials in blow molding operations to enhance their barrier properties. A treatment gas containing from about 0.1 to 10% by volume of fluorine in an inert gas was injected into the parison to inflate or expand it into shape. Due to the higher temperature, a combined blowing and reaction time of approximately 5 seconds was utilized at which time the parison was cooled and the reactive gas and container recovered.
Commercially fuel tanks having enhanced resistance to hydrocarbon permeation have been marketed under the Airopak trademark wherein the fuel tanks are produced by utilizing blow molding techniques. In these processes the parison is initially conformed to the desired shape by inflating or expanding with an inert gas, followed by purging of the parison and subsequent injection of the parison with a reactive gas containing from 0.1 to 104 fluorine. The reactive gas is removed from the parison, recovered and the container ejected from the mold.
There have been substantial modifications to the early processes for the production of containers having enhanced barrier properties via blow molding. Some of the processes are described in the following patents:
U.S. Pat. No. 4,142,032 discloses an apparent improvement in the Dixon et al., process utilizing a reactive gas containing both fluorine and bromine at temperatures below the softening point of the polymer and pressures one atmosphere or less. Basically, then, the '032 process is similar to that of Joffre '468 in that the fluorination/bromination is effected at low temperature, thus requiring long reaction times.
U.S. Pat. No. 4,404,256; 4,264,750; and 4,593,050 disclose a low temperature fluorination of polyolefins, e.g., polyethylene and polypropylene, to form low energy surfaces utilizing wave energy in association with the fluorination process. The. '256 and '750 patents disclose contacting the polymer surface with ions or radicals comprising fluorine or fluorinated carbon in a cold plasma. The '050 discloses fluorination of polymer surfaces utilizing a fluorinating gas and enhancing the fluorination by exposing the surface to ultraviolet radiation to assist in the fluorination process.
U.S. Pat. No. 4,701,290 discloses the production of high density polyethylene fuel tanks having increased barrier resistance to hydrocarbon solvent and vapor permeation via off-line fluorination. The key to enhancing barrier permeation resistance lies in the precise control of fluorination of the polyethylene fuel tank and this is achieved by passing the treatment gas through a container filled with aluminum oxide. By measuring the quantity of oxygen generated from the Al.sub.2 O.sub.3, one controls the concentration of fluorine contained in the treatment gas and thereby controls the level of fluorine acting upon the surface of the container within a predefined reaction time.
U.S. Pat. No. 5,073,231 discloses an off-line fluorination process for producing plastic objects with a smooth surface finish. According to this patent, smooth surfaces are produced by treating plastic objects at elevated temperatures with a mixture of fluorine and an oxidizing agent. Fluorinated surfaces thus produced are claimed to (1) incorporate very low fluorine in the plastic (less than 6 .mu.g/cm.sup.2) and (2) provide good barrier action against non-polar solvents.
Improvements in blow molding processes have also been made since the discovery of the Dixon, et al. higher temperature blow molding process and these are reported in U.S. Pat. No. 4,830,810; 4,617,077; and 4,869,859. The '810 patent discloses a blow molding process for producing containers comprising inserting the preform or parison into the mold, injecting inert gas into the parison at a first level of pressure to expand and conform it to the shape of the mold, raising the pressure to a level above the first level by injecting a fluorine containing reactant gas into the parison after pressure tightness has been determined. The reactant gas typically comprises a mixture of fluorine and nitrogen with the fluorine concentration being about 1% by volume; the injection pressures are from 4 to about 10 bar and reaction times of about 30 seconds. The process eliminates some of the hazards associated with blow molding using a reactive gas to conform the parison to the mold since pressure tightness at the time of injection of the reaction gas may not have been established. The '077 patent modifies the '810 process in that inflation of the parison in the mold with an inert gas is conducted at high pressure followed by treatment of the interior of the parison with a fluorine containing gas at substantially lower pressure than that used for initially expanding the parison or preform. After reaction, the reaction gas is replaced with a flushing and cooling gas at a pressure substantially higher than the pressure of the reaction gas and even higher than the initial injection gas used to preform the parison.
U.S. Pat. No. 4,869,859 discloses a blow molding process for the preparation of high density polyolefin fuel tanks. The patentees indicate that severe wrinkling of the thermoplastic occurs at temperatures close to or above the melting point, causing an increase in the permeation rate of the solvent. According to this patent, fluorination is carried out at temperatures from 50.degree. to 130.degree. C., preferably 80.degree. to 120.degree. C., and below the melting temperature of the polymer, in an effort to achieve uniform temperature distribution and fluorination of the interior surface of the material.
U.S. patent application having Ser. No. 07/985,665 filed Dec. 3, 1992 discloses a multi-step blow molding process for producing permeation resistant thermoplastic containers. A thermoplastic parison is expanded within a closed mold by means of an inert gas, the parison vented and then treated with a reactive gas containing 0.1 to 1% fluorine by volume while the parison is at a temperature above its self supporting temperature for a time sufficient to effect fluorination of the interior surface of the parison. Subsequently, the interior surface of the pre-fluorinated parison is treated with a reactive gas containing at least twice the initial concentration but not less than about 1% fluorine by volume for a specified time to form the fluorinated parison with increased permeation resistance.